The present invention relates to process for the decomposition of fluorine-containing compounds, in particular perfluoroalkanes, a catalyst for use in such a process for preparing the catalyst.
Fluorine-containing compounds represent a group of compounds which can display a high level of stability, making them difficult to convert or decompose. As a result, these compounds can be difficult to remove from effluent and waste gas streams before such streams are released into the atmosphere. A particularly troublesome group of compounds to decompose or convert are the perfluoroalkanes.
Perfluoroalkanes are a specific group of halogen-containing compounds that consist of carbon and fluorine atoms. Perfluoroalkanes are saturated, that is they do not possess double or triple carbon-carbon bonds. Perfluoroalkanes differ from chlorofluorocarbons (CFC""s), hydrochlorofluorocarbons (HCF""s) and hydrofluorocarbons (HFC""s) in that perfluoroalkanes contain neither hydrogen nor chlorine. Examples of operations which emit perfluoroalkanes to the environment include, but are not limited to, electrolytic aluminum smelting, manufacture of fluoroplastics, dry chemical etching, chamber cleaning process and semiconductor manufacturing process. Examples of perfluoroalkanes include carbon tetrafluoride (CF4) hexafluoroethane (C2F6, octafluoropropane (C3F8), octafluorocyclobutane (C4H8) and decafluorobutane (C4H10). Perfluoroalkanes are some of the most stable compounds known (Kiplinger et al. Chem. Rev., p.373 (1994). As a result, these compounds are efficient as so-called xe2x80x9cgreen housexe2x80x9d gases, with global warming potentials estimated to be many times that of CO2 (Langen et al., 1996). The stability of perfluoroalkanes makes these compounds difficult to decompose or convert to useful products, such as for example by conversion of perfluoroalkanes to perfluoroalkenes.
A number of catalysts and catalytic processes have been reported for the decomposition of halogen-containing organic compounds. A review of the literature reveals that the majority of these catalysts and catalytic process focus on the decomposition of chlorine-containing compounds, or organic compounds containing both chlorine and fluorine. Only limited results have been reported on the decomposition of organic compounds containing only fluorine. Bond and Sadeghi, in an article entitled xe2x80x9cCatalyzed Destruction of Chlorinated Hydrocarbons,xe2x80x9d J. Appl. Chem. Biotechnol., p.241 (1975), report the decomposition of chlorinated hydrocarbons over a platinum catalyst supported on high surface area alumina. In particular, experiments are reported on the conversion of a range of chlorinated hydrocarbons over a platinum/gamma-alumina catalyst on which a hydrocarbon fuel is being oxidized. The ability of this catalyst to decompose perfluoroalkanes was not reported.
Kamaker and Green, in an article entitled xe2x80x9cAn investigation of carbon difluoridedichloride (CF2Cl2) decomposition on TiO2 Catalyst,xe2x80x9d J. Catal, p394 (1995), report the use of a TiO2 catalyst to decompose CFCl2 at reaction temperatures between 200 and 400xc2x0 C. in streams of humid air. Again, there is no discussion of the decomposition or conversion of perfluoroalkanes.
Bickel et al., in an article entitled xe2x80x9cCatalytic Destruction of Chlorofluorocarbons and Toxic Chlorinated Hydrocarbons,xe2x80x9d Appl. Catal., B:Env. p.141 (1994), report the use of a platinum supported on phosphate-doped zirconium oxide for the decomposition of CFC113 (Cl2FCCC1F2) in air streams. The catalyst was able to achieve in excess of 95% decomposition of CFC113 at reaction of 500xc2x0 C. for approximately 300 hours of continuous operation. After this time, the catalyst rapidly deactivated. The ability of this catalyst to decompose perfluoroalkanes was not reported.
Fan and Yates, in an article entitled xe2x80x9cInfrared Study of the Oxidation of Hexafluoropropene in TiO2,xe2x80x9d J. Phys. Chem, p. 10621 (1994), report the decomposition of a perfluoroalkene over TiO2. Perfluoroalkanes differ from perfluoroalkanes in that they contain a carbon-carbon double bond, that is are unsaturated. Although the catalyst was able to readily decompose hexafluoropropylene (C3F6), the loss of titanium, as TiF4, was evident. The formation of TiF4 would undoubtedly lead to deactivation of the catalyst. The ability of this catalyst to decompose perfluoroalkanes was not reported.
Faris et a., in an article entitled xe2x80x9cDeactivation of a Pt/Al2O3 Catalyst During the Oxidation of Hexafluoropropylene,xe2x80x9d Catal. Today, p. 501 (1992), report the decomposition of hexafluoropropylene over a catalyst comprising platinum supported on a high surface area alumina. Although the catalyst could readily decompose hexaflurorpropylene at reaction temperatures between 300 and 400xc2x0 C., over the course of the experiment (less than 100 hours), the aluminum oxide was converted to aluminum triflouride, which resulted in a severe loss of catalytic activity. The ability of this catalyst to convert perfluoroalkanes was not reported. However, transformation of the aluminum oxide to aluminum triflouride suggests that aluminum oxides will not be stable in fluorine-containing environments.
Campbell and Rossin, in a paper entitled xe2x80x9cCatalytic Oxidation of Perfluorocyclobutene over a Pt/TiO2 Catalyst,xe2x80x9d presented at the 14th N. Am. Catal. Soc. Meeting (1995), report the use of a platinum/TiO2 catalyst to decompose perfluorocyclobutene (C4F6) at reaction temperatures between 320 and 410xc2x0 C. The authors note that even at a reaction temperature of 550xc2x0 C., no conversion of perfluorocyclobutane (c-C4F8), a perfluoroalkane, could be achieved using the Pt/TiO2 catalyst. Results presented in this study demonstrate that perfluoroalkane are significantly more difficult to decompose or convert than the corresponding perfluoroalkane.
Nagata et al, in a paper entitled xe2x80x9cCatalytic Oxidative Decomposition of Chlorofluorocarbons (CFC""ss) in the Presence of Hydrocarbons,xe2x80x9d Appl. Catal. B:Env., p. 23 (1994), report the decomposition of 1,1,2-trichloro-1,2,2-trifluoroethane (CFC113), 1,2-dichloro-1,1,2,2-tetrafluoroethane (CFC114) and chloropentafluoroethane (CFC115) in the presence of hydrocarbons using catalyst prepared by impregnating a range of support materials, in particular gamma-alumina, silica-alumina, zeolites, mordenite, ferrierite, with vanadium, molybdenum, tungsten and platinum. Silica was also employed as a catalyst. Tungsten (VI) oxide and vanadium (V) oxide catalyst exhibited the highest activity. The decomposition of the CFC""s became more difficult as the number of carbon atoms in the CFC molecule decreased. The ability of the catalyst to decompose or convert perfluoroalkanes was not reported. However, results suggest that the parent molecule becomes more difficult to destroy upon replacement of chlorine with fluorine. No information regarding the stability of the catalyst was reported.
Burdeniuc and Crabtree, in an article entitled xe2x80x9cMineralization of Chlorofluorocarbons and Aromatization of Saturated Fluorocarbons by a Convenient Thermal Process,xe2x80x9d Science, p. 340 (1996), report the transformation of cyclic perfluoroalkanes to perfluoroarenes by means of contact with sodium oxalate to yield sodium fluoride as a reaction product. Both reactions, however, are slow and non-catalytic, since sodium oxalate is stoichiometrically consumed by being converted into sodium fluoride during the course of the reaction. This process would not be able to decompose or convert perfluoroalkanes present in streams of air, since the oxygen and/or moisture in the air would readily convert the sodium oxalate to sodium oxide.
Accordingly, it can be seen that there exists a need for a process for the conversion of perfluoroalkanes present in effluent gaseous streams, in particular streams containing perfluoroalkanesin a mixture with air. Pending U.S. patent application Ser. No. 08/662129 concerns a process for the decomposition of perfluoroalkanes in which the perfluoroalkanes are contacted with a catalyst comprising aluminum oxide. The aluminum oxide is preferably stabilized by the presence of an element selected from barium, calcium, cerium, phosphorus, chromium, cobalt, iron, lanthanum, magnesium, nickel, tin, titanium and zirconium. The catalyst is prepared by preparing a slurry of a single source of aluminum oxide, specifically pseudoboehmite, and peptizing using an acid, such as nitric acid, formic acid or acetic acid. The resulting mixture, including one or more stabilizers if desired, is then dried and calcined. A very high activity of the catalyst in the decomposition of perfluoroalkanes is demonstrated in the specific examples of the specification.
In addition to high activity, a most important aspect of any catalyst, particularly a catalyst for use on a commercial scale, is the stability of the catalyst and its ability to maintain a high level of activity for extended periods of operation. Surprisingly, it has been found that the stability of an aluminum oxide-based catalyst in the decomposition of perfluoroalkanes can be markedly increased by employing aluminum nitrate as the aluminum oxide source.